The present invention relates to external gear type fluid displacing machines, especially to external gear type hydraulic pumps or motors, and more particularly to improvements in the construction of housing and bearing members for the trunnions of spur, helical or herringbone gears in such machines.
In presently known external gear type pumps or motors (hereinafter referred to as pumps for convenience), the bearings members which surround the trunnions of gears have circular cylindrical peripheral surfaces and the housing or body of the pump has circular cylindrical internal surfaces which are immediately adjacent to the peripheral surfaces of the bearing members. It has been found that such bearing members are subjected to excessive wear which is most pronounced in the low-pressure region (i.e., in the case of a pump in the region adjacent to the inlet port). The end faces of the gears rub against the adjacent end faces of the bearing members and cause the latter to wear away after relatively short periods of use. The reason for such excessive wear upon the end faces of bearing members in immediate proximity to the gears is that the bearing members undergo a pronounced deformation when the load thereupon increases, i.e., when the gear teeth convey fluid from the inlet port toward the outlet port. The deformation is readily detectable in the low-pressure area and results in radial upsetting of bearing members. Such upsetting, in turn, results in arching of those portions of the end faces of bearing members which are adjacent to the respective end faces of associated gears at the low-pressure side of the pump. The wear progresses until each end face of each gear is in full surface-to-surface contact with the adjacent end face of the respective bearing member. The arching of the end faces of bearing members in the low-pressure area is enhanced by end pressure which develops as a result of more or less pronounced flexing of trunnions under load. It has been found that the useful life of bearing members having circular cylindrical peripheral surfaces and being surrounded by circular cylindrical internal surfaces of the pump body is extremely short so that such bearing members must be inspected and replaced at frequent intervals. Defective bearing members affect the output of the pump and are likely to cause damage to or destruction of other pump components.